This invention relates to fluid operated power tools and, more particularly, to a new and improved shut-off valve for such tools.
One area of use of the present invention is in tightening threaded fasteners, although the principles of the present invention can be variously applied. Fluid operated power tools for such use typically are provided with a shut-off valve to shut off the supply of motive fluid to the tool automatically when the desired torque is reached. An important consideration in the design of such tools is the capability defined in terms of the amount of torque variation of the tool run on a variety of jobs from hard to soft. On hard jobs, the tool stops suddenly, and inertial energy from high speed motor elements tends to increase final torque, while on soft jobs inertial energy is dissipated in heat during tool deceleration. In the power fastening art, a practical limit for a hard job is one in which the fastener goes from loose to tight in a turn of thirty degrees, and the practical limit for a soft job is one in which the fastener goes from loose to tight in two revolutions or more. That is to say, after two revolutions of tightening, inertial effects on a power-driven fastener are negligible. Such inertial effects increase exponentially as tightening angle is decreased. Very rarely are threaded joints designed to reach desired torque in less than 30 degrees. Accordingly, the shut-off valve of such fluid-operated power tools will encounter variations in fastening jobs which will affect fastener torque at tool shut-off.
It would, therefore, be highly desirable to provide a fluid-operated power tool including a shut-off valve having improved capability in substantially matching the torque achieved on hard jobs with the torque achieved on soft jobs. In addition, it would be advantageous to provide such a tool having reduced impulse or kick at shut-off and wherein peak torque is relatively close to set torque, with less variation. The foregoing desirably should be achieved by a shut-off valve of relatively simple construction which is efficient and effective in operation.